The present invention relates to an electrode sheet for an electric double-layer capacitor, a method for its manufacturing, a polarizable electrode and an electric double-layer capacitor using the polarizable electrode. Specifically, the present invention focuses on the application of polarizable electrode to an electric double-layer capacitor of rolling type and the like, which require a process of rolling or bending of the polarizable electrode.
An electric double-layer capacitor, which has high capacitance of farad class, excellent characteristics of charge/discharge cycle and the capability of undergoing rapid charge, has been used for a backup power supply of electronic component, an onboard battery for a vehicle (an energy buffer) and the like.
An electric double-layer capacitor is briefly described referring to FIG. 1.
FIG. 1 is a sectional view showing the main structure of an electric double-layer capacitor.
As shown in FIG. 1, an electric double-layer capacitor 101 includes a casing 102 housing a pair of carbon electrodes (polarizable electrodes) 104 which interposes a separator 103, and a pair of collectors (elements) 105. And the casing 102 is filled with an ion conductive electrolytic solution. The electric double-layer capacitor 101 employs electric charges (shown by + and xe2x88x92 in FIG. 1) as dielectrics in a normal capacitor, which are generated at an interface between the solid carbon electrodes 104 and the liquid electrolytic solution, and spaced at a distance of molecule.
Electrolytic solutions used for electric double-layer capacitors are roughly categorized into an aqueous electrolytic solution made of a dilute sulfuric acid added with an electrolyte and an organic electrolytic solution made of an organic solvent added with an electrolyte. An appropriate type of electrolytic solution is selectably applied to an electric double-layer capacitor taking into account its usage. An electric double-layer capacitor using an aqueous electrolytic solution is advantageous in terms of internally lower electric resistance and higher power density. On the other hand, an organic electrolytic solution, which allows a higher withstand voltage per cell, is advantageous in terms of energy density. It also allows selection of inexpensive and light metals such as an aluminum alloy for a casing.
A method for manufacturing an electrode sheet is generally used, which has a process of mixing an electrochemically active material such as activated carbon, an electrically conductive filler such as carbon black and a binder such as polytetrafluoroethylene (PTFE).
It is required of the electrode sheet, which is applied to an electric double-layer capacitor in a form of rolled electrode, to possess high efficiency in terms of voltage maintenance within a predetermined voltage or self discharge over a long period of time.
For example, Japanese Published Patent application 2001-267187 reports that efficiency for voltage maintenance was improved by controlling the tensile strength of an electrode. Also Japanese Patent Publication 07-105316 (Japanese Published Patent Application 63-107011) shows that porosity or Gurley number was selected as a parameter so as to improve the performance of an electrode sheet.
However, since the method reported in the former document, which introduced only the tensile strength for the improvement, was not able to guarantee the durability for deformation, an amount of falling particles tended to increase. In addition, pressure acting locally on a separator affected the efficiency for voltage maintenance adversely.
On the other hand, the parameter selected by the latter document did not sufficiently explain the interaction between an electrode and an electric double-layer capacitor.
The present invention provides an electrode sheet which is applied to an electric double-layer capacitor in a form of a rolled element or an element subjected to bending process, and its manufacturing method. The electrode sheet not only is resistant to a crack but also has high efficiency in terms of voltage maintenance. Also the present invention provides a polarizable electrode made of electrode sheets, which has high efficiency for voltage maintenance, and an electric double-layer capacitor.
As a result of study, it has been discovered that the problems described above can be solved by controlling the coefficient of elongation for a polarizable electrode within a predetermined range.
An aspect of the present invention provides an electrode sheet for an electric double-layer capacitor. The electrode sheet is molded from granules which are produced from ingredients including an electrochemically active material, an electrically conductive filler and a binder. And the electrode sheet is bonded with a collector foil so as to form a polarizable electrode which is rolled or bent so as to be applied to the electric double-layer capacitor. In this aspect of the present invention, a coefficient of elongation S for the polarizable electrode is adapted to be greater than (R+T)/R and less than or equal to 1.11, where R represents a curvature of an inscribed circle at a bent portion of the polarizable electrode and T represents a thickness of the polarizable electrode.
The electrode sheet described above not only is resistant to a crack while it is under a process of rolling or bending but also has high efficiency in terms of voltage maintenance. The term xe2x80x9ccoefficient of elongationxe2x80x9d is intended to mean a coefficient of elongation at the fracture point relative to the original length.
Another aspect of the present invention provides an electrode sheet, in which a contact angle is equal to or less than 100 degrees when the contact angle is defined as (180-ALPHA) degrees, where ALPHA represents an apex angle of a droplet of an electrolytic solution for an electric double-layer capacitor, and when the droplet lies on the electrode sheet.
The electrode sheet described above allows manufacturing of an electric double-layer capacitor that has lower electric resistance and stable performance.
Still another aspect of the present invention provides a method for manufacturing an electrode sheet for an electric double-layer capacitor. The electrode sheet is molded from granules which are produced from ingredients including an electrochemically active material, an electrically conductive filler and a binder. And the electrode sheet is bonded with a collector foil so as to form a polarizable electrode which is rolled or bent so as to be applied to the electric double-layer capacitor. The method includes the following steps:
(a) kneading the ingredients so that the binder is subjected to fibrillation, and molding a lump out of the ingredients after the fibrillation;
(b) crushing the lump into granules for the electrode sheet of the electric double-layer capacitor; and
(c) forming the granules into the electrode sheet.
In this aspect of the present invention, one of a period of time and strength of kneading at the step (a) is adjusted so that a coefficient of elongation S for the polarizable electrode can be greater than (R+T)/R and less than or equal to 1.11, where R represents a curvature of an inscribed circle at a bent portion of the polarizable electrode and T represents a thickness of the polarizable electrode.
The method described above can provide the electrode sheet, which is resistant to a crack during its rolling or bending process, for the electric double-layer capacitor having high efficiency in terms of voltage maintenance.
In this connection, the period of time and strength of kneading correlated with the coefficient of elongation S which satisfactorily falls in (R+T)/R less than Sxe2x89xa61.11 have been demonstrated by an experiment or a simulation based on data obtained by the experiment. The conditions applied to the experiment, which include ingredients, a mixing ratio, a thickness of electrode sheet and a method of manufacturing, are fixed for the experiment and simulation.
Yet another aspect of the present invention provides a method, in which one of the period of time and the strength of kneading at the step (a) is adjusted so that a contact angle can be equal to or less than 100 degrees when the contact angle is defined as (180-ALPHA) degrees, where ALPHA represents an apex angle of a droplet of an electrolytic solution for the electric double-layer capacitor, and when the droplet lies on the electrode sheet.
The method of the present invention described above can provide the electrode sheet for the electric double-layer capacitor, which is resistant to a crack and has high efficiency in terms of voltage maintenance and low electric resistance as well as stable performance.
In this connection, the period of time and the strength correlated with the contact angle less than (180-ALPHA) degrees have been demonstrated by an experiment or a simulation based on data obtained by the experiment. The conditions applied to the experiment, which include ingredients, a mixing ratio, a thickness of electrode sheet and a method of manufacturing, are fixed for the experiment and simulation.
It is preferable to select classified granules having a particle diameter of 47-840 micron meters for manufacturing the electrode sheet according to the present invention.
The granules selected this way prevent fine particles of carbon and electrically conductive filler from separating or desorbing from the electrode sheet, so that they may not float in an electrolytic solution or exist between separators. It will enable manufacturing of the electric double-layer capacitor, which has high efficiency in terms of voltage maintenance.
A further aspect of the present invention provides a polarizable electrode for an electric double-layer capacitor, which includes an electrode sheet molded from granules that are produced from ingredients including an electrochemically active material, an electrically conductive filler and a binder, and a collector foil which is bonded with the electrode sheet directly or via a layer of an adhesive so as to form the polarizable electrode which is rolled or bent so as to be applied to the electric double-layer capacitor. In this aspect, a coefficient of elongation S for the polarizable electrode is adapted to be greater than (R+T)/R and less than or equal to 1.11, where R represents a curvature of an inscribed circle at a bent portion of the polarizable electrode and T represents a thickness of the polarizable electrode.
The polarizable electrode for the electric double-layer capacitor described above not only is resistive to a crack during rolling or bending process but also has high efficiency in terms of voltage maintenance as well as the electrode sheet according to the present invention.
A still further aspect of the present invention provides an electric double-layer capacitor including the polarizable electrode described above.
The electric double-layer capacitor according to this aspect has high efficiency in terms of voltage maintenance.